Immunopathologic and serologic studies of rheumatic valvular disease have indicated an immunologic reaction involving valvular connective tissue and valve fibroblasts in the pathogenesis of this disease. It is proposed to define the reactive constituents of heart valves employing sera of rheumatic fever patients, antisera to human heart valves, and antisera to group A streptococcal fractions. Data have been presented indicating that C-reactive protein reacts with phosphoryl choline-containing C-polysaccharide, lecithin, and sphingomyelin with activation of human complement via the classical pathway. The significance of these observations for the function of CRP in inflammation will be examined.